1. Field of the Invention
The present invention pertains in general to the spatial frequency relationship between the optical elements of a flat-panel display.
2. Description of the Prior Art
Devices for enlarging optical images using optical waveguides are well known. Early devices included bundles of optical fibers that were tapered along their length such that the fibers were larger in cross-section at one end than at the other. To produce an enlarged image, the original image was input into the small end of the fiber bundle. The image that emerged from the large end of the fiber bundle was magnified by a factor equal to the ratio of the diameter of the large end to that of the smaller end. While producing the desired effect, these devices often produced poor quality, distorted images because of the difficulty in producing uniform tapers in the large number of fibers required by the devices.
Another technique for producing enlarged images using optical waveguides involved cutting a bundle of optical waveguides at a bias and redirecting the emergent light in a direction normal to the bias cut. As a result of the bias cut, the image emerging from the bundle of waveguides was magnified in one direction. Glenn, Jr. discloses one such technique in U.S. Pat. No. 4,208,096.
In U.S. Pat. No. 3,402,000, Crawford also discloses a fiber optic image enlarger that employs the bias-cut waveguide bundle technique. The Crawford image enlarger includes a first bundle of optical fibers having a bias cut that magnifies an input image in a first direction. The magnified image is then redirected into a second bundle of optical fibers having a bias cut that magnifies the input image in a second direction. The resultant image, therefore, is magnified in two dimensions.
While successful in producing enlarged images, devices based on the prior art typically exhibit problems originating from the inefficient cooperation between the optical structures. A first problem can result from nonuniform spacing between the optical waveguides or from slight variations in alignment of optical waveguide structures. Normally, manufactured bundles of optical waveguides include a certain amount of nonuniformity in the spacing between the individual waveguides. This nonuniformity generally goes unnoticed when images are transmitted through a single bundle of optical waveguides. When multiple bundles of optical waveguides having similar spatial frequencies are coupled together, however, the nonuniformity in spacing or misalignment of the fibers of each bundle causes Moire fringe patterns that appear as alternating light and dark areas in the transmitted image.
A second problem, may result from inadequate spatial sampling of the optical elements of each optical structure. Typically, light input image devices include a structure comprising an array of pixels each projecting a portion of the input light image. The maximum resolution state of such a device includes adjacent pixels alternating between light and dark. When the input image device is optically coupled to a bundle of optical waveguides each having a dimension similar to the pixel dimension, any misalignment between the pixels and optical waveguides can significantly degrade the output image. Rather than each adjacent waveguide transmitting alternating light and dark light signals in a one-to-one correspondence with the pixels of the input imaging device, each waveguide will transmit light from a portion of a dark pixel and a portion of an adjacent light pixel. At the far end of the optical waveguides, the light and dark components of the light input to each waveguide merge together. As a result, the resolution of the input image is lost, and the output image appears gray.
While in theory the pixels of the input imaging device could be aligned perfectly with each corresponding optical waveguide to provide accurate mapping and transmission of the input image, in practice such a result is not feasible. Manufacturing of such a device would be extremely difficult and prohibitively costly.
These problems are further compounded in devices, such as the Crawford device, that include multiple light transmission interfaces. In these devices, the product of the light transmission characteristics through each optical structure and at each light transmission interface determines the overall quality of the output image. Therefore, any global nonuniformity in either bundle of waveguides or any misalignment of optical structures at the light transmission interfaces serves to compound the negative effects on the output image.